As a technology which estimates a direction of a face in an image and which is related to the present invention, there are technologies described in patent document 1 and patent document 2, for example.
For example, a face direction estimation processing technology described in patent document 1 acquires frontal-face data of a plurality of people in advance, creates an average face, posts the average face onto a three-dimensional model, and generates an image by rotating by optional inclined angles. The face direction estimation technology described in patent document 1 estimates the direction of a face of an input image by using determination of an image of an inclined angle that has the highest correlation degree with the input image. The face direction estimation technology described in patent document 1 premises that an input image is a facial image. Patent document 2 also takes this premise similarly.
As above, in a face direction estimation technology, whether an input image is a facial image or not is judged in advance in many cases. Judgment of whether an input image is a facial image or not is performed based on detecting a face in an input image using a face detection technique described in non-patent document 1, for example.
Various facial detection techniques such as one seen in non-patent document 1 extract an area of a face from a whole image for which detection of a face is desired. Specifically, the facial detection techniques extract various partial images from a whole image. Next, the facial detection techniques judge whether an extracted partial image is an image in which a face shows up mainly or an image that is not such one. Then, the facial detection techniques judge an area corresponding to an image which has been determined as an image in which a face shows up mainly as an area where a face exists.
Further, hereinafter, “an image on which a face shows up mainly” is called a facial image and “an image that is not so” is called non-facial image for convenience of description.
Technologies described in such as non-patent document 1 learn processing for detecting a face after a lot of facial image groups and non-facial image groups have been prepared in advance. For example, a facial image group used for learning is acquired by designating an area where a face exists from an image including the face based on manpower and clipping the area.
As the case with the above-mentioned patent document 1, many of face direction estimation technologies premise that facial detection processing has been performed using the related facial detection techniques or the like. That is, in many cases, a face direction estimation technology and a facial detection technique are independent technologies. In many cases, a face direction estimation technology premises that whether an image of an estimation target is a facial image or a non-facial image is already known. On the other hand, a facial detection technique premises that the direction of a face in an image of a detection target is known roughly.
Here, the technology described in non-patent document 2 improves accuracy of the both processing not by performing a face direction estimation processing and a facial detection processing as individual, but by performing simultaneously.
The technology described in non-patent document 2 prepares a great deal of facial image groups and non-facial image groups in advance. About each image of each prepared image group, the technology described in non-patent document 2 gives information whether it is a facial image or not and, if it is a facial image, information in which direction it faces together in advance. Then, the technology described in non-patent document 2 learns facial detection processing and face direction estimation processing simultaneously using data made by integrating each image and its information. Therefore, the technology described in non-patent document 2 can perform facial detection processing and face direction estimation processing simultaneously and with a high degree of accuracy.